The present invention relates to linear actuators and in particular to a linear actuator of the screw thread type.
Hitherto linear actuators have been provided in which an externally threaded shaft engages an internally threaded member, the velocity ratio, that is the ratio of number of turns of the driven component to the number of pitch lengths of linear travel of the other component of such systems being one. It is possible, by increasing the diameter of the internally threaded member, so that the threads thereon engage the threads of the shaft member at only one point, to provide a linear actuator with an increased velocity ratio. However, because the teeth of the threaded members only engage at one point, the load carrying capacity of such actuators are significantly reduced. Furthermore, loading of the members is non-symmetrical and in order to accommodate these non-symmetrical loads, the complexity of construction of the actuator is increased. For example, with such actuators, it is normal to utilise a plurality of internally threaded members arranged to engage the shaft member at angularly spaced locations.